As scientists consider whether human cloning can be safe, the stories of two
sheep, one famous and one dead, illustrate the dream and the danger.
One tale centers on the photogenic Dolly, the first animal ever cloned from an
adult mammalian cell (SN: 3/1/97, p. 132: http://www.sciencenews.org/sn_arc97/3_1_97/fob1.htm). While investigators continue to study
the 5-year-old sheep for late-developing abnormalities, such as premature aging,
Dolly has given birth to normal lambs and is by all accounts healthy. She may be a
bit overweight, but that’s because reporters have fed her so much, jokes Alan
Colman of PPL Therapeutics in Edinburgh, Scotland, which funded the creation of
Less well known and lacking a cute public name was a cloned ewe born not too long
after Dolly. It had no obvious physical abnormalities at birth and was an active
lamb, but it panted all the time, recalls Ian Wilmut of the Roslin Institute in
Edinburgh, whose group cloned both Dolly and the second ewe. The respiratory
problem was so severe that researchers within a few weeks decided to euthanize the
hyperventilating animal. An autopsy showed that its lungs had not developed
Wilmut says that this second ewe’s fate ought to make those who would clone people
think again. “Who would be responsible for a child born with an abnormality like
that?” he asks.
Although Dolly’s birth has inspired a few maverick researchers to want to make
human cloning a reality, the often-disastrous results of animal cloning have
convinced many scientists that an effort to clone a person is unthinkable at this
“Based on the plausible outcomes, it’s ridiculous to move forward with human
cloning,” says Don Wolf of the Oregon Regional Primate Research Center in
Beaverton, who is working to clone monkeys (see “Are cloned monkeys next?,” below). “It’s totally
In August, a scientific and frequently emotional discussion on the feasibility of
human cloning played out in public at the National Academy of Sciences (NAS) in
Washington, D.C. (SN: 8/18/01, p. 105: Cloning hearing creates media frenzy). There, before a panel considering whether
to recommend a ban on human cloning, more than a dozen scientists described their
successes and failures at cloning mice, sheep, goats, and cows. As they discussed
possible explanations for what goes wrong, the scientists often focused on
evidence of abnormal gene activity during clones’ development. And almost all
concluded that cloning a person would be unsafe.
But not everyone did. Three proponents of human cloning defended their plans and
vowed to continue. They argued that scientists have more knowledge about
reproduction in people than in most other species, and people may not be
susceptible to some of the problems that have arisen in cloned animals.
“We need to proceed with human cloning,” says Brigitte Boisselier, a chemist and
director of a cloning company formerly called Clonaid, whose location she refuses
to reveal. “I believe it’s a fundamental right to reproduce the way you want.”
Litany of problems
At the meeting and in scientific publications, researchers have documented a
litany of problems that plague animal cloning. Many of the cloned embryos develop
so abnormally that they don’t even make it out of the petri dish alive.
Despite its failure rate, the cloning process is straightforward (SN: 4/5/97, p. 214: http://www.sciencenews.org/sn_arc97/4_5_97/bob1.htm). First, investigators obtain an egg cell from an animal and remove its
nucleus, the sac containing almost all the egg’s DNA. They replace that nucleus
with one from a cell of the animal they wish to copy. Usually, this is done by
fusing the nucleus-lacking egg with the donor cell.
Finally, a jolt of electricity or some other stimulus tricks the egg into dividing
as if it had been fertilized by a sperm. Once the growing embryo has reached a
multicell stage known as a blastocyst, it’s ready to be transferred into the
uterus of a surrogate mother.
Jonathan Hill of Cornell University, who has cloned cattle, notes that about one-third of the cloned embryos that are implanted don’t survive even the first month
of a cow’s normal 9-month gestation period. Of those that do, another half die in
the next month or two, apparently because of abnormal placental development. This
prenatal die-off continues through birth.
“The placenta is not supplying nutrients, and the fetus starves,” says Hill.
The same pattern of spontaneous abortions holds true for cloning in other species.
“The losses are extraordinarily large and happen at all stages of gestation,” says
Making it to birth is no guarantee that a clone will survive. Hill notes that
newborn cloned calves frequently emerge in bad shape. Some have skeletal
abnormalities. Many suffer a variety of lung and heart problems.
Hill estimates that 25 to 50 percent of clones are oxygen-deprived at birth. Some
can be saved, but many die.
Cloned cows, sheep, goats, and mice also often display what scientists call large-offspring syndrome. Internal organs, limbs, and overall body are over-sized, and
the newborns are sickly. The large fetuses can also place a mother at risk during
Colman, however, says that the sorry state of cloned animals has been exaggerated.
At the August meeting, Colman presented data from several research
groups–including his own–showing that in some cases, 95 to 100 percent of cloned
pigs, cows, and sheep that made it to birth were thriving. “Many cloned animals
are healthy,” he says. “But because most of us here abhor the idea of human
reproductive cloning, there’s an extrapolation of results to indicate that [we]
never get any healthy clones.”
Colman also offered the scientists a sense of cloning’s efficiency by comparing it
with human in vitro fertilization (IVF). He noted that many human embryos created
through IVF, like cloned embryos, don’t make it to the blastocyst stage.
In one set of published data, only 8 to 12 percent of the human embryos created
with IVF resulted in a live birth, he says. Some groups that are cloning cattle
have achieved comparable efficiencies, Colman noted.
When cloning fails
So, what exactly goes wrong when cloning fails? Because Dolly and some other
cloned animals have begotten normal offspring, scientists don’t think that cloning
introduces permanent mutations into an animal’s genes.
Consequently, biologists have begun to focus on the regulation of gene activity in
cloned embryos. When a nucleus from an adult cell, say a skin cell, is placed
inside an egg cell, its DNA must undergo dramatic changes before it’s ready to
create a new animal. Skin-specific genes must turn off, for example, and genes
that drive embryonic development must begin to turn on, each one at exactly the
Scientists refer to this transformation as the reprogramming of the nucleus.
Incomplete reprogramming is the main reason that cloned embryos fail so often,
Several recent studies have suggested a problem with methylation, a chemical
modification that usually shuts down gene activity. As cells begin to specialize
into adult tissues, methylation seems to inactivate genes that are no longer
needed. For the DNA in an adult nucleus to guide the development of a clone, its
existing methylation pattern must return to an embryonic state.
In the June Nature Genetics, however, South Korean scientists reported that
methylation patterns in cloned bovine embryos are frequently quite different from
those observed in normal embryos.
A few months earlier, a research group that included some of Dolly’s creators
focused attention on a gene encoding a protein called insulin growth factor 2
receptor (IGF2R). The gene normally shows a trait known as maternal
imprinting–only the copy inherited from the mother is active in an offspring.
There are also paternally imprinted genes, and scientists believe that methylation
plays a large role in maintaining the imprinted status of a gene. The parental
gene that is methylated lies dormant, leaving the other parent’s copy of the gene
active in the offspring.
The Roslin Institute’s Lorraine Young and her colleagues examined the activity of
the IGF2R gene in sheep fetuses created through IVF, which like cloning often
results in large-offspring syndrome in sheep. Compared with normal-size fetuses,
those showing signs of the syndrome had 30 to 60 percent less activity of the gene
the team reported in the February Nature Genetics. Moreover, DNA regulating the
gene’s activity showed less methylation than normal.
Since IGF2R normally plays a growth-suppressing role within the fetus, the
scientists concluded that reduced activity of its gene could be responsible for
large-offspring syndrome both in IVF and cloned embryos.
There are about 40 known imprinted genes in a person. In general, genes in which
only the paternal copy is active promote fetal growth, while genes in which just
the maternal copy is active limit it (SN: 5/15/99, p. 312).
The profound influence of imprinted genes on fetal growth has made them prime
suspects for many of the developmental abnormalities that afflict cloned animals.
Some scientists have even looked at these genes in cloned animals that show no
Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge,
Mass., and his colleagues recently examined seemingly normal adult mice that had
been cloned by placing the nucleus from an embryonic stem cell into a mouse egg.
The researchers observed inconsistent methylation and activity among the handful
of imprinted genes that they tested, not including that for IGF2R, Jaenisch
reported at the NAS meeting and in the July 6 Science.
“Even apparently normal clones have an abnormal regulation of many genes,” he
contends. “Completely normal clones may be the exception.”
Other scientists questioned this conclusion. They note that cloning typically uses
a nucleus from an adult cell, not from an embryonic stem cell. Therefore, the
genetic chaos Jaenisch observed may not be pertinent to the more common methods of
Jaenisch “has laid down the gauntlet for us to prove this,” says Colman.
Primates are different
Even as investigators probe the reasons that animal cloning fails so often,
another question has arisen: How relevant is that research to human cloning? After
all, although animals created through in vitro fertilization sometimes suffer
large offspring syndrome, there’s no evidence that human test-tube babies do.
Randy L. Jirtle of Duke University Medical Center in Durham, N.C., and his
colleagues have now added a new element to the debate. In the Aug. 15 Human
Molecular Genetics, the investigators report that the gene for IGF2R is not
imprinted in primates, even though it is in rodents, pigs, sheep, and other
animals that researchers have cloned. So, people have two active copies of this
growth-suppressing gene instead of just one.
Jirtle’s team is focusing on the finding’s implications for cancer, but it might
explain why people are less susceptible than some animals to fetal overgrowth.
“This marked species difference in IGF2R gene imprinting indicates that humans may
be easier to clone than nonprimates,” Jirtle says, who adds that he’s not
advocating such attempts.
Yet abnormal activity of the IGFR2 genes may not be the only factor contributing
to large-offspring syndrome. Indeed, Jaenisch’s group has observed that some
oversize mouse clones have normally imprinted IGFR2 genes.
In addition to Boisselier’s company, which is affiliated with the UFO cult known
as the Raelians, two individuals have announced that they plan to collaborate on
an effort to clone people. Severino Antinori of the International Associated
Research Institute in Rome has a long history of work in assisted reproductive
technology. He recently received both recognition and condemnation for helping
postmenopausal women, such as a 63-year-old grandmother, become pregnant with the
use of donated eggs. Antinori intends to work with reproductive physiologist
Panayiotis Michael Zavos of the Adrology Institute in Lexington, Ky.
Zavos argued at the August meeting that the animal data present an overly
pessimistic view of cloning for several reasons. The animals cloned often come
from inbred strains, which he speculates make cloning more difficult. Furthermore,
the cloned animal embryos chosen for implantation, Zavos says, do not undergo any
kind of screening process to weed out the ones unlikely to succeed.
Antinori adds that animal cloners may not be using the optimum culture conditions
for initially growing the embryos.
By applying decades of expertise in IVF, it’s possible to make cloning as safe and
efficient for people as are other reproductive technologies, Antinori and Zavos
contend. They point out that some IVF physicians already screen human embryos by
plucking out one cell and checking it for specific mutations before placing the
rest of the embryo in a woman’s uterus.
Furthermore, in vague comments that Boisselier declined to explain more fully, she
suggested to the NAS panel that her research group had developed ways to guarantee
cloning safety by checking the on-off status of imprinted genes in a human embryo.
Reproductive biologist Alan Trounson of the Monash Institute in Clayton, Australia
immediately derided Boisselier’s claim as “ludicrous.”
First, he says, imprinted genes are not the only genes misregulated in clones.
Second, he and most of the scientists at the NAS meeting agreed, today’s
technology isn’t advanced enough to check dozens or hundreds of genes at one time.
When physicians now do preimplantation diagnosis, it typically focuses on
identifying mutations in a single gene. Finally, many of the genes that may cause
a problem don’t become active until after the implantation of an embryo.
“At present, there is no way to predict whether a given clone will develop into a
normal or abnormal individual,” Jaenisch concludes.
The desire to guarantee that a human clone be healthy may reflect a philosophical
difference between people favoring and opposing attempts at human cloning.
Pointing to the high rates of spontaneous abortions and birth defects that plague
natural pregnancies, Zavos argues that human reproduction is intrinsically risky.
Of course, more than safety arguments enter discussions of human cloning. Some
people object to cloning out of religious, ethical, or moral principles. Indeed,
abortion politics has become so entangled in considerations of any form of human
embryo research that bioethicist R. Alta Charo of the University of
WisconsinMadison told the NAS panel that “the United States is almost incapable
of a sensible policy discussion in this area.”
Still, in the next few weeks, the NAS panel plans to release recommendations to
guide legislators as they consider regulation of human cloning. A moratorium or an
outright ban on human cloning seems likely, and several countries have already
asked the United Nations to pass such a restriction.
Are cloned monkeys next?
The birth of the sheep named Dolly provoked an international furor about the
possibility of human cloning. Don Wolf worries that the world will similarly
overreact if he and his colleagues clone an adult nonhuman primate, such as a
That fear “is a disincentive to continue, but [we] simply can’t be intimidated,”
he says. “There’s such a need for genetically identical monkeys, such as
for AIDS-vaccine work, that we need to press on.”
Several years ago, Wolf’s team at the Oregon Regional Primate Research Center in
Beaverton successfully cloned monkeys by using the nucleus of an embryonic cell
(SN: 3/8/97, p. 142). Yet it hasn’t succeeded when starting with the nucleus of a
cell from an adult monkey. Indeed, the researchers are still struggling to get
such cloned embryos ready for transfer into a surrogate mother.
“Progress has been slow and has been limited. We’re trying to establish conditions
where we can get [cloned] embryos to grow to the implantation stage. Once we do
that with a reasonable degree of regularity, we’ll go back to doing embryo
transfers to establish pregnancies,” Wolf told Science News. “We’re on the cusp.”